(1) Field of the Invention
The present invention relates to a process for the preparation of a polyester hollow formed body and also to a vessel prepared according to this process. More particularly, the present invention relates to a process in which simultaneously with draw-blow-forming of a polyester preform to a hollow formed body, heat setting of the molecular orientation can be performed efficiently. Furthermore, the present invention relates to a polyester vessel having novel molecular orientation characteristics and being excellent in resistance to contraction.
(2) Description of the Prior Art
A biaxially draw-blow-formed vessel of a thermoplastic polyester such as polyethylene terephthalate has not only excellent transparency and surface gloss but also impact resistance, rigidity and gas-barrier properties required for bottles, and therefore, this vessel has been used for bottling various liquids.
However, polyester vessels are generally defective in that the heat resistance is poor, and when contents are hot-filled, thermal deformation or contraction of the volume is readily caused. Accordingly, many methods for heat-setting biaxially draw-blow-formed vessels after the forming operation have been proposed as means for eliminating this disadvantage.
As heat-setting methods, there are known, for example, a method in which a formed body obtained by draw-blow-forming is taken out from a draw-blowing mold and then held in a heat-setting mold and the formed body is heat-set, as disclosed in Japanese Patent Publication No. 56606/85, and a method in which heat setting is carried out simultaneously with draw-blow-forming in a blow-forming mold, as disclosed in Japanese Patent Publication No. 6216/84. Furthermore, Japanese Patent Application Laid-Open No. 53326/82 teaches a method in which a heat treatment is carried out simultaneously with draw-blow-forming in a primary mold and the formed body is taken out from the primary mold and blow-formed in a secondary mold directly without cooling.
Of the foregoing known methods, the method in which heat setting is carried out simultaneously with draw-blow-forming in a blow-forming mold is industrially excellent because the number of steps is small and the apparatus cost is low. However, the method is still insufficient in that the manufacturing speed is low because a relatively long residence time in the mold is necessary for heat setting after the draw-blowing operation and cooling for withdrawal of a hollow formed body.
As means for overcoming this disadvantage, there has been proposed a method in which the hollow-forming mold is maintained at a temperature as high as possible within a range where the final hollow formed body can be taken out without deformation substantially under non-cooling, for example, at 100.degree. C., and a polyester preform is biaxially drawn simultaneously with blowing of high-temperature high-pressure air in the polyester preform (see Japanese Patent Application Laid-Open No. 95666/79). According to this method, it becomes unnecessary to elevate and drop the temperature of the mold, but in case of conduction of heat from the high-temperature gas, because of the presence of the heat transfer boundary film, a relatively long time is still necessary for completion of heat setting and the resistance to thermal contraction is not satisfactory.
In the process for the preparation of a hollow formed body in which draw-blow-forming and heat setting of the molecular orientation are simultaneously carried out, it is expected that if a preform being draw-formed is maintained at a temperature as high as possible, heat setting will be possible while the preform is being draw-formed and the residence time of the hollow formed body in the mold will be drastically shortened. However, preheating of an amorphous preform at a high temperature results in occurrence of troubles such as thermal deformation of the preform and thickness unevenness at the draw-forming step. Moreover, whitening and reduction of the drawability are caused by thermal crystallization of the polyester. Therefore, this means cannot be practically adopted.